Do you mean the big cylindrical concrete lumps at/on the bridge in Garscube estate ?Surely that's what you're referring to as they are the only 'corrugated' 'stones' in there that I'm aware of. I was told that their purpose was anti-tank bollards dating back to WW2 !One of the 'custodians/security guards' who used to walk about with a single barrel shotgun told me that in the early '80s !(I lived very nearby)....Maybe he was at the wind-up ?! (The old mill site at the foot of Dawsholm road isn't part of Garscube estate).....

Oooops !!!!I just had a look on Google earth and the things I mentioned have been removed Maybe they moved them and put/dumped them where you've found them !Upload a pic to Flickr and post the link, it sounds like the same things as the sides of them were 'corrugated'....